The objectives of this proposal are to determine the susceptibility profiles for many of the bacterial species encountered in subgingival plaque samples from patients with destructive periodontal diseases and to measure the concentrations of antibiotics locally achieved in gingival fluid. The antimicrobial susceptibilities of approximately l000 strains reprersenting 35-40 species of bacterial will be determined to ten potentially useful antibiotics for periodontal therapy. Susceptibility testing will be performed using an agar dilution technique employing an agar medium specifically developed for antimicrobial susceptibility testing of periodontal isolates. Concentrations of the antibiotics, phenoxymethylpenicillin, erythromycin base, erythromycin stearate, and clindamycin, will be measured in gingival fluid and compared with blood levels to determine if significant differences exist. Clindamycin and phenoxymethyl-penicillin will be measured by the use of extremely sensitive agar diffusion assays developed for this purpose. A similar assay will be developed for measuring erythromycin. Profiles of susceptibility and resistance established for the bacterial species and the antibiotic concentrations achieved in gingival fluid will be combined to aid in the selection of chemotherapeutic agents appropriate for the control of different microbial complexes associated with distinct forms of periodontal disease.